The conventional cooling fan generally includes a pallet, a hub and blades; the blades are integrated with the hub through plastic injection molding. For saving material, the thickness of the hub is reduced to minimum thickness; therefore, the hub is relatively thin. When large blades are connected to the hub, the roots of the blades need to be distorted before connecting to the hub. However, such structure easily results to a fracture at the root of the blade. For improving connection strength between the blades and the hub, some solutions have been proposed. For example, the patent application with Publication No. WO2008/141253A1, published on Nov. 20, 2008, discloses a fan structure comprising a hub member, a plurality of blade members extending radially outward from said hub member, a plurality of helical gusset members. The numbers of gusset members correspond to the number of blade members, and each of said gusset members extends from the hub member adjacent to one blade member to the trailing edge of an adjacent blade member. The technical solution of the above application improves the connection strength between the blades and the hub through providing gusset members. Although such gusset members are able to improve the connection strength, such connection structure easily results to stress concentration; thus results to a reduction of the strength of the blade, and fracture of the blade. Meanwhile, such structure can result to a reduction of air quantity and efficiency of the blades.